Application of the European Directive on sewage disposal seeks to limit the amount of non-treated wastewater that is discharged into the natural environment. In particular, treatment stations should fully purify all collected water, with the exception of water due to exceptional rain events. In addition, greater reliability of works is to be required. Effluent discharged into the natural environment should be of constant quality even when the wastewater to be treated varies very considerably both with respect to pollution load and with respect to flow rate. It is therefore necessary to keep the purification station in the best possible operating state, and to adapt operating criteria on a permanent basis in order to respond to any event that modifies the operating conditions of the biological treatment.
A large number of purification stations making use of activated sludge have only one treatment basin in which carbon pollution is eliminated and in which nitrification is performed, at least in part. Satisfying the European Directive makes it necessary, in certain cases, to achieve total nitrification and denitrification. All three treatment operations may be performed in a single basin, which then operates alternately in an aeration stage for eliminating carbon and for nitrification, and in an anoxia stage for denitrification.
Unfortunately, one of the main causes of misfunction in stations of this type is poor matching of oxygen supply whether because insufficient oxygen is supplied or because the availability of oxygen over time is inappropriate, thereby giving rise to poor results from the treatment. It is also possible to end up by losing sludge as can happen after spontaneous denitrification in the clarifier situated downstream from the biological treatment basin, and by suffering phenomena of anaerobiosis in the treatment basin due to insufficient oxygen in certain zones, thus giving rise, in the long term, to the appearance of filamentary bacteria and then of foam.
Devices have therefore been developed to regulate aeration in such treatment basins, but they still suffer from various drawbacks.
The use of cyclical dispensing equipment or "clocks" provides a solution in part only since such devices are set for a pre-established mean level of operation without adapting to variations. There is thus, for example, a serious risk of anaerobiosis leading quickly to damage of the biological medium.
The use of sensors measuring the dissolved oxygen or the oxidation-reduction potential, together with various combinations of sensors, serves to detect floor values, with a high threshold enabling aeration devices to be stopped, and a low threshold enabling aeration devices to be restarted. Timing devices are also used in case such thresholds are not reached.
In order to optimize nitrification and denitrification reactions, it is essential for oxygen to be supplied when necessary and in sufficient quantity, and not merely to supply oxygen, as is the case in previously described systems.
A first improvement in existing systems consists in continuously measuring the oxidation-reduction potential of the medium and in analyzing the shape of the curve showing this potential as a function of time:
EH=f(time), cf. 2nd International Specialized Conference on Design and Operation of Small Wastewater Treatment Plants, Trondheim, Norway, 28-30 June 1993, "Automatic Regulation of Activated Sludge Aeration", by F. Lefevre et al.
Analysis is performed by calculating the derivative and studying the derivative. If the derivative is positive, that means the oxidation-reduction potential is rising. If the derivative is negative, that corresponds to the oxidation-reduction potential falling in the anoxia stage. When the derivative is zero, then a stabilization stage is taking place.
The system then calculates the duration of aeration or non-aeration to be provided, which duration is equal to the duration of aeration or non-aeration necessary for bringing the oxidation-reduction potential to the required value for performing either carbon elimination or nitrification or denitrification, plus the additional amount of time required for actually performing the reaction.
Such a system nevertheless suffers from drawbacks since the curve of oxidation-reduction potential can be asymptotic in appearance (derivative tending to zero). Unfortunately, at certain values of the oxidation-reduction potential, it is absolutely essential to avoid switching to a quasi-stabilization stage since although conditions are not necessarily catastrophic for preserving the biomass, they are nevertheless inappropriate for the desired treatment.